1. Field of the Invention
The present invention relates to contact lenses that are used in treating hyperopia and presbyopia, and more particularly, to contact lenses that are shaped to provide gradual altering of the patient's cornea during continued wear to reshape the cornea to reduce the hyperopic and presbyopic condition.
2. Description of the Related Art
Many people experience difficulties with their vision due to a number of possible conditions. The most common vision problem is a condition known as myopia or nearsightedness. Myopia is a common condition where an eye cannot focus on far-away objects because the cornea of the eye is curved too steeply (i.e., where the radius of curvature of the cornea is smaller than normal) to provide adequate focusing at the retina of the eye. Another condition is known as hyperopia or farsightedness. With hyperopia, the eye cannot focus on both far and near objects because the curvature of the cornea of the eye is too flat (i.e., where the radius of curvature of the cornea is larger than normal)to provide adequate focusing at the retina of the eye. Hyperopia is common among young children. Severe hyperopia will induce lazy eye or amblyopia in childhood. Mild or moderate hyperopia is tolerable and insidious in young ages but will cause reading problems in older age. Another common problem is astigmatism, where unequal curvature of one or more refractive surfaces of the cornea prevents light rays from focusing clearly at one point on the retina, resulting in blurred vision. Presbyopia is the most common vision problem in adults 40 years and older. It does not matter whether they are emmetropic, myopic or hyperopic in far vision, the middle-aged population over 40 years old will begin to experience difficulty in focusing on close objects, due to the loss of flexibility of the eye's crystalline lens. Presbyopia may occur and complicate other refractive problems such as hyperopia, myopia or astigmatism.
Hyperopia and presbyopia are both conditions for which no entirely suitable permanent treatment has been developed. The conventional way is to wear a pair of heavy convex glasses. One approach to correcting hyperopia is through laser surgical reshaping of the cornea. However, such surgical procedures have not been entirely safe and there have been less favorable vision outcome for the hyperopia surgery than that of myopia surgery.
Another approach to treating some or all of these conditions is to alter the corneal shape by wearing contact lenses which are designed to continually exert pressure on selected locations of the cornea to gradually force or mold the cornea into the desired normal corneal curvature. A retainer lens is then worn on a part-time basis to prevent the cornea from returning to its previously deformed shape. This method of treatment is commonly referred to as orthokeratology (referred to herein as “ortho-k”). The success of any treatment by ortho-k is dependent upon the shape and structure of the contact lens. For example, conventional contact lenses with a longer central radius of curvature than the central radius of the cornea are known to change the shape of the cornea by compressing the surface at its apex. This reshaped cornea has a lengthened radius of curvature in its central zone, which serves to improve myopia. Although it has been well known to treat myopia using modem Ortho-K lenses to reshape the cornea, treating hyperopia or presbyopia using Ortho-K lenses has been met with uncertain outcome and results.
Ortho-k has been performed in some form or another since the early 1970s. There are three factors that impact the effectiveness and desirability of ortho-k procedures and lenses. The first factor is the time. needed to achieve a desired visual correction. Unfortunately, the time needed to achieve a desired visual correction using conventional ortho-k techniques and lenses has been a serious problem, since it will take several months, or even years, for small amount of hyperopia to be reduced. The second factor is the amount of hyperopia that can be corrected using ortho-k. Conventional ortho-k techniques and lenses were limited to a reduction of no more than about one diopter of hyperopia. The third factor is the amount of time that the correction would “hold” before degrading (known as the “maintenance period”). Conventional ortho-k techniques and lenses provide variable length for the maintenance period. To prolong this maintenance period, a patient would have to wear a retainer lens.
Some patients have been fitted with a series of progressively steeper regular RGP lenses to treat hyperopia. The lens diameter was small (7–8 mm) and the base curve steeper than the central cornea curvature. Lenses would need to be replaced upon the showing of some effect, while the results are usually minimal and unpredictable. The lenses sometimes became tightened to cause adverse effect.
U.S. Pat. No. 5,963,297 to Reim and U.S. Pat. No. 5,349,395, No. 4,952,045, No. 5,191,365, No. 6,010,219 to Stoyan disclose Ortho-k lens designs for myopia reduction. There has been no lens specifically designed for increasing the curvature power of the cornea for hyperopia reduction. Orthokeratology for the presbyopia has never been addressed or mentioned before, largely because it was thought that no lens could mold a cornea into dual shapes for clearing up near, as well as far, vision simultaneously. The conventional way to treat a myopic, and yet presbyopic, person by Ortho-k is either to sacrifice the far vision of both eyes (under-corrected) or to sacrifice monocular far vision (Mono-vision). Both ways make it hard to be accepted by most of the patients.
Notwithstanding the improvements provided by modern Orthokeratology for myopia, there remains a need for a contact lens that can be used for effective ortho-k of hyperopia and/or presbyopia.